Imager having photosensitive material contains polymorphous silicon

ABSTRACT

Assembly of sensors formed as an imager with a detection brick including a photosensitive material, a brick for addressing and optionally processing signals from the sensor(s), an interconnection brick located between the detection brick and the addressing brick, this brick including connection pads, characterized in that the photosensitive material of the detection brick contains polymorphous silicon. 
     The invention also relates to a method for the making of the latter.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority based on International PatentApplication No. PCT/FR03/01965, entitled “Imager” by Cyril GUEDJ, JoseALVAREZ, Yvan BONNASSIEUX, Jean-Paul KLEIDER, Norbert MOUSSY, Pere ROCAI CABARROCAS and Svetoslav TCHAKAROV, which claims priority of FrenchApplication No. 02 07893, filed on Jun. 25, 2002 and French ApplicationNo. 02 07894, filed on Jun. 25, 2002, and which were not published inEnglish.

TECHNICAL FIELD

The invention is in the field of photosensitive sensors, in particularin the visible wavelength range, i.e. approximately between 400 and 800nm and in the ultraviolet wavelength range, i.e. approximately between10 and 400 nm. These sensors include a layer possibly consisting ofseveral sublayers of a photosensitive material converting photons intoelectric charges. These sensors may exist in an isolated form or in theform of an assembly of sensors forming together an imager. The inventionnotably relates to sensors or a PIN or a NIP diode imager.

STATE OF THE PRIOR ART

A photosensitive sensor delivers an electrical signal, the amplitude ofwhich, in an operating range, is an increasing monotonous function ofthe intensity of the light which it receives. Sensors are generallyarranged in the form of a matrix of sensors formed with one or severalcolumns, and with one or several lines.

UV sensors are generally arranged in the form of isolated sensors fordetecting the presence of UV radiation or an assembly for UV imaging.

A matrix including several lines and columns is used in most imageforming devices. Other configurations are known, in particularconfigurations wherein the pixels are arranged in polygonal structures,i.e., structures in which the sensors of the pixels occupy apexes ofpolygons, relatively to each other, for example regular triangles orpentagons or hexagons. These structures are used in particular forincreasing the number of pixels per unit surface or for producing groupsof pixels sensitive to different colors or to different wavelengths.Sensors which form together an imager are currently called “pixels”because each sensor then delivers an electrical signal corresponding toone pixel of an image to be formed.

References of the prior art documents which will be quoted in theapplication are recalled at the end of the description with the numberbetween square brackets which is assigned to them in the presentdescription.

U.S. Pat. No. 6,114,739 [1] describes the general shape ofphotosensitive sensors, the sensitive material of which is formed by anamorphous material. In the description of the prior art made in thispatent, a general form of an assembly of sensors forming together animager is described. This description is repeated hereafter inconjunction with the appended FIG. 1 which reproduces FIG. 1 of thispatent.

This figure illustrates a cross-section of a network of sensorscomprising a sensitive material also called active material.

The network of sensors is formed on a substrate 10. The substrate isgenerally of the CMOS (complementary metal oxide silicon) or BiCMOS(bipolar complementary metal oxide silicon) or bipolar type. Thesubstrate may also include a great diversity of technologies, includingCCD (charge coupled devices) type technologies.

An interconnection structure 12 connects this substrate to an n dopedlayer 14 which is above the interconnection structure 12. This layer 14is used for forming with intrinsic i 16 and p doped 18 layers stackedover the layer 14, a network of photodetecting PIN diodes. The positionof the diodes formed from layers 14, 16 and 18 is determined by theposition of via conductors 20 formed as a matrix network in theinterconnection structure 12.

A translucent conducting layer 24 is positioned above the matrix networkof PIN diodes formed from layers 14, 16 and 18. This layer 24 forms anelectrode for biassing the PIN diodes.

The substrate layer 10 generally includes a circuit for sensing thesignal delivered by each diode and circuits for addressing and for ananalog or digital processing of these signals.

Operation is as follows. An inverse bias voltage is applied between theelectrode formed by the layer 24 and the substrate 10. The therebygenerated electric field generates a current corresponding to thedisplacement of electric charges generated by photons which penetrateinto the active material. The current from the sensitive material zoneswhich are found straight above a via 20, is preferentially directedthrough this via 20 towards a signal sensing circuit. Each of the vias20 thereby receives a current which is a function of the illuminationreceived by the sensitive material surface found above this via 20. Thedifferent signals are then processed by the addressing and processingcircuits included in the substrate 10 for example.

As explained in U.S. Pat. No. 6,114,739 [1], a limitation of the imagesensor structure which has just been described comes from the fact thatthe different individual sensors, the signal of which corresponds to onepixel of the image to be formed, are not electrically insulated fromeach other. In particular, a current formed according to the lightreceived by a surface found straight above a first via 20 may very wellleak towards one of the neighboring consecutive vias 20. In terms ofimage, each of the pixels trickles onto the neighboring pixels so thatthe pixels of an image formed from the imager are not clearly separatedfrom each other. A first method for reducing this flaw as explained inthe description of the prior art, appearing in U.S. Pat. No. 6,114,739[1], consists of providing slots in the p or n layer 14 found above thevias 20. These slots provide better individualization of each of thediodes found in the alignment of a via 20, and accordingly, betterseparation of the pixels of an image.

U.S. Pat. No. 6,114,739 [1] describes an improvement of this prior artintended for improving separation. This improvement is described inconjunction with FIG. 2 of this patent. Each detecting diode is furtherindividualized by the fact that it is equipped with its own electrode.FIG. 2 of the drawings appended to the present application reproducesFIG. 2 of this patent [1] while keeping the reference numbers assignedin FIG. 1 for the components with the same function as those in FIG. 1.In the embodiment of FIG. 2, the n or p layer 14 added in dotted linesin FIG. 2 appended to the present application, is etched afterdeposition, so that only portions 44 found above a via 20 are allowed tosubsist. Moreover, a contact conduction layer is also depositedimmediately above the interconnection structure 12. This contact layeris etched in same way as the n layer 14. An unremoved portion 45 of thislayer forms with a remaining portion 44 of the n layer 14, an electrodeof a PIN or NIP diode. Thus, as the n layer 14 is cross-linked, each ofthe diodes contributing to the delivery of the signal for forming animage pixel is individualized in a better way than in the prior art.

The photosensitive material forming the sensitive layer 16 of the diodesis an amorphous material, for example amorphous silicon or amorphouscarbon, or amorphous carbonaceous silicon or amorphous germanium or evenamorphous germanium-silicon. It is specified that this list is notexhaustive. A way of achieving the produced structure in FIG. 2 isdescribed in this patent in conjunction with FIGS. 3–7 of this patent,which each illustrate the structure at different stages of itsproduction.

The significance of a structure such as illustrated in FIG. 1 or 2, isthat it splits up the detection portion from the addressing,interconnection, signal processing portion. Thus, layers 14, 16, 18, 24together form a detection brick 50. The substrate 10 and the connectionand possibly the signal processing circuits associated with thissubstrate form a processing brick. Both of these bricks areinterconnected through the interconnection brick 12. With this modularapproach, highly integrated compact low cost imagers may be made, i.e.,which include a large number of individual detectors per unit surfaceand use little current.

A drawback of this type of structure, wherein the active array is formedby an amorphous material, is that the performances are poor in terms ofsignal dynamic range and maintaining performances with ageing. Forexample, amorphous silicon is known to have a large density of defects,of the order of 10⁶ per cm³. These defects cause a limitation of thepossible operating rate because they induce retention of the deliveredsignal and therefore of the image. These structures are thus unsuited toa use for producing successive images with a high repetition frequency.Finally, the structure regularly deteriorates during the illuminationtimes, which is detrimental to the service life of the imager. Moreover,it incurs a high risk of deterioration or even destruction, under strongilluminations.

Reference article [2] reports different existing UV detectors.

Conventionally, the semiconductor detectors used for UV detection areP-I-N diodes in crystalline silicon. Recently, materials of the GaAlNand SiC type have been used for producing UV detectors, insensitive tovisible light (visible-blind) and to solar radiation (solar-blind).

U.S. Pat. No. 5,682,037 [3], filed in 1996, describes an exemplaryembodiment of a SiC-based UV sensor. In the description of the prior artmade in this patent, the following is first stated at the beginning ofcolumn 2: “Crystalline silicon photodiodes have optimal efficiency inthe visible range and can only be used for UV detection aftersophisticated and costly optical and mechanical treatments. They requirelow voltage power supplies and may be arranged into a network withdimensions of a few centimeters”.

UV CCD devices are components based on crystalline silicon which alsorequire a very special treatment. These are multichannel detectors whichare very sensitive and have a high signal-to-noise ratio, in particularwhen they operate at low temperature. The photogenerated electrons arecollected in an array of pixels and are then read out sequentially. Itis thus possible to reconstruct a two-dimensional image. There are atleast three major disadvantages in UV detection by CCD devices, i.e.,cost, the impossibility of obtaining two-dimensional CCD arrays withlarge dimensions and the requirement of filtering visible light or otherradiations when the UV radiation to be detected needs to be detected ona background of visible light and other radiations.

The invention described in patent [3] is directed to solving the problemof filtering visible and infrared radiations, of electric powerconsumption, of large scale integration and additionally for a lessercost.

The described invention provides optimization of the thickness andabsorption coefficient of the layer of amorphous conductors forming thejunction, as well as the geometrical shape of a metal grid used as frontelectrode. In addition to increasing efficiency so that it is maximum inthe UV range, the invention provides adjustment of the operating rangeof the detector so as it makes it capable of detecting near or far UVradiation depending on the application. It has already been demonstratedthat by acting on the deposition parameters and on the impurityconcentration in the silicon alloy, absorption may be optimized forvisible light or infrared light. The physical parameters to be optimizedare the absorption profile and thickness of the detector. Thisoptimization may be obtained by controlling the deposition parameters,i.e., notably deposition time and carbon percent in the alloy.

Optimization and reproducibility of the thickness of the layers are madepossible by controlling the radio frequency discharge (glow discharge)of the coating. The absorption coefficient provided by hydrogenatedamorphous silicon, itself depends on the fundamental properties of thematerial, such as the width of the forbidden band of the semiconductorand the density of states in the band. The latter in turn depend on thegrowth parameters of the layer in a very complicated way. A simple andreproducible way for varying the profile of the absorption coefficientversus the wavelength is to form silicon/carbon or silicon/germaniumalloys with known percentages. This is achieved by introducing into thedeposition chamber, a controlled flow of gaseous methane or germane,respectively. The obtained carbon/silicon alloy is an amorphoussemiconductor, with a forbidden band of higher energy than amorphoussilicon, which penalizes absorption of visible and infrared light ascompared with UV light.

However, the obtained a-SiC alloy should not contain a too high carbonpercentage, relatively to silicon, because its electronic propertieswould suffer therefrom.

In conjunction with FIGS. 1 a and 1 b of this patent [3], the generalembodiment of the invention of patent [3] is described.

FIG. 1 a illustrates a top view of a sensor as described in [3] and FIG.1 b illustrates a cross-section of said sensor.

The sensor comprises from bottom to top, as illustrated in FIG. 1 b, asubstrate for example in glass, but, preferably in quartz, in order tolet ultraviolet radiation to pass through it. It then includes atransparent conductor 4, and an n⁺ doped hydrogenated amorphous siliconlayer. The sensitive material is formed by two sublayers of hydrogenatedamorphous carbonaceous silicon (a-SiC: H), one being n⁺ doped and theother p⁻ doped, finally, by an amorphous hydrogenated carbonaceoussilicon p⁺ layer 2, covered with a conducting grid 5. It is pointed outthat it is preferable to work at zero bias between electrodes 4 and 5.

The difficulty consists in maintaining electro-optical performancesunder illumination as the amorphous material tends to deteriorate underUV illumination. Moreover, the external quantum efficiency (capabilityof converting photons into electric charges) is low from the nature ofthe material.

In the case of diodes based on crystalline silicon, growth is achievedat a very high temperature (above 400° C.), which makes it incompatiblewith direct deposition on the CMOS readout circuit on substrate Si.

SUMMARY OF THE INVENTION

The object of the present invention is a light sensor in particular forthe visible wavelength range, unique or arranged in an assembly ofsensors forming together an imager. The present invention is alsodirected to allowing a UV imager to be produced on a circuit, forexample a CMOS circuit, by the fact that the sensitive layer may bedeposited at a lower temperature, for example of the order of 300° C. orless.

A light sensor optionally arranged in an assembly of sensors formingtogether an imager, according to the present invention, further has theadvantages of the modular systems described above:

enhanced time response, i.e., the rise and fall times of the value ofthe signal delivered according to the variations of the illumination,more closely following the variations even when these variations arefast,

low image retention, and therefore allowing successive images to beproduced with a high repetition rate,

better resistance to ageing, and increased resistance to strongilluminations.

Moreover, the imagers made with the sensors, are adapted to largeintegration, i.e., they operate properly even with a large number ofpixels per unit surface.

In particular, the invention proposes a strong increase of sensitivityfor wavelengths between 10 and 400 nm, while reducing sensitivity toparasitics around 700 nm.

Finally, with an imager according to the invention, it is possible touse a larger range of readout circuits, in particular including readoutcircuits with switching of bias between each readout. This possibilitybecomes open because of the large time dynamic range of thephotosensitive material.

For all these purposes, the invention is related to a unique sensor oran assembly of visible light sensors, formed as an imager, each sensordelivering a signal corresponding to one pixel of the image, and having,

a detection brick with a detection zone including a photosensitivematerial,

a brick for addressing and possibly processing signals from thesensor(s), this brick notably having an addressing circuit and,

an interconnection brick located between the detection brick and theaddressing brick, this brick having connection pads connecting pixels ofthe imager to the addressing circuit, so-that the signals from thepixels are individualized,

characterized in that the photosensitive material of the detection brickcontains at least one polymorphous silicon layer.

The invention is also related to a sensor or an assembly of sensors forultraviolet radiation, each sensor delivering a signal corresponding toone pixel of the image, and having,

a detection brick with a detection zone including a photosensitivematerial,

a brick for addressing and possibly processing signals from the sensorsand,

an interconnection brick located between the detection brick and theaddressing brick

characterized in that the photosensitive material of the detection brickcontains polymorphous silicon with a thickness less than 0.4 μm andpreferably between 0.01 and 0.05 μm.

Selectivity for the sensitive frequency range may be determined byselecting a temperature for depositing the polymorphous silicon layer,between 150° C. and 250° C., with a pressure between 1,300 and 1,800mTorrs.

Polymorphous silicon has a low density of states for defects in themiddle of the forbidden bands and a high product of the carrier mobilitytimes the lifetime of said carriers. On this subject, reference may bemade to reference article [4], the reference thereof appearing at theend of the document. Consequently, the time dynamic range, i.e., thecapability of a detector made in this way, of following time variationsof an illumination, is improved at high illumination variation orelectric bias rates. Image retention is reduced and it therefore becomespossible to produce successive images with a high repetition rate.Moreover, it was seen that the performances of an imager made with astructure according to the invention had better stability over time,notably under strong illuminations.

Preferably, the interconnection brick is formed by pads embedded intothe insulator.

Preferably, the interconnection pads are either in aluminium or copperor tungsten.

Preferably, the insulating material embedding the pads is formed by astack of dielectric layers forming Bragg mirrors.

Preferably, electrodes are formed above the pads, these electrodeshaving a lower surface electrically coupled with a pad, and an uppersurface, the upper surface of the electrode having a larger surfacedimension than the lower surface in contact with the pad.

Preferably, the upper surface of the electrodes has a shape of a cup.

Preferably, a lower portion of each electrode is embedded into aninsulator layer, an upper portion of this electrode appearing above saidinsulator layer.

Preferably, the insulator layer surrounding a lower portion of theelectrodes is formed by a stack of layers forming Bragg mirrors.

Preferably, the electrodes are either in aluminium, copper or tungstenor titanium or titanium nitride or chromium or a doped semiconductor oran organic conductor or a conducting oxide or finally even a compositestack of the materials mentioned above.

Preferably, the layer of polymorphous material is placed above the layerincluding the insulator and the electrodes.

Preferably, at least an upper silicon layer has a lower portioncontaining carbon and an upper portion containing boron.

Preferably, the thickness of the polymorphous material layer of a sensoror an assembly of sensors for visible light is between 0.5 and 2 μm.

Preferably, the detection zone including polymorphous silicon is anintrinsic area of a PIN or NIP diode.

The invention also relates to a method for making an assembly ofphotodetectors according to any of the embodiments of this assembly,characterized in that:

after making a substrate, notably including an addressing circuit, andoptionally signal processing circuits,

one or more layers of insulating materials are deposited,

said layer is etched in order to form holes in this layer,

some holes are filled with conducting material thereby forminginterconnection pads (5, 5′),

mechano-chemical polishing is carried out optionally,

one or more sublayers of insulating material are deposited,

said insulating material sublayer is etched above pads,

a layer is deposited above the etched insulator layer, thereby forming anon-planar layer of conducting material, cups appearing above the pads,

the conducting material layer which has just been deposited, is etchedin order to form electrodes separated from each other,

an unintentionally doped polymorphous material layer is deposited,

a doped layer is deposited,

finally, a conducting material layer forming an upper electrode isdeposited.

Preferably, the temperature for depositing the polymorphous material isbetween 175° C. and 250° C. and the pressure for deposition is between1,300 and 1,800 mTorrs.

Preferably, deposition of the polymorphous material is followed bydeposition of a layer containing carbon, at least in its lower portion.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with the help of the appendeddrawings wherein:

FIG. 1, already described, shows a cross-sectional view of a structureof a photodetector according to the prior art;

FIG. 1 a, already described, shows a top view of a structure of aphotodetector according to the prior art;

FIG. 1 b, already described, shows a cross-sectional view of saidphotodetector of the prior art illustrated in FIG. 1 a;

FIG. 2, already described, shows a cross-sectional view of anotherphotodetector of the prior art;

FIG. 3 is a schematic illustration of the structure of polymorphoussilicon;

FIGS. 4–6 show different ways for characterizing polymorphous silicon;

FIG. 4 illustrates the exodiffusion spectrum of polymorphous silicon;

FIG. 5 illustrates curves of the infrared absorption spectrum;

FIGS. 6 a and 6 b, respectively illustrate a high resolutiontransmission electron microscopic image (HRTEM) of a polymorphousmaterial and its spatial Fourier transformed equivalent to an electrondiffraction spectrum;

FIG. 7 illustrates an embodiment of the invention which corresponds toFIG. 2, and wherein the photosensitive material is polymorphous silicon;

FIG. 8 illustrates a particular embodiment of the invention according totwo alternatives;

FIG. 9 illustrates curves of residual dark current divided by the photoncurrent versus optical power, for amorphous silicon on the one hand, andfor polymorphous silicon on the other hand.

FIG. 10 illustrates in an orthonormal reference system, values for theleak current surface density for different sizes of pixels.

FIG. 11 illustrates an embodiment of the invention wherein thephotosensitive material is polymorphous silicon;

FIG. 12 illustrates a top view of a detector according to the inventionwherein the upper electrode is formed with two interdigitated fingercombs:

FIG. 13 illustrates in an orthonormal reference system, values of theleak current surface density for different sizes of pixels.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

As explained above, as compared with the prior art, the inventionconsists of replacing an amorphous material forming the active layerwith polymorphous silicon.

First of all, it will be pointed out how it is possible to recognizepolymorphous silicon from amorphous silicon upon examining them. Thestructure of polymorphous silicon is schematized in FIG. 3. Polymorphoussilicon includes a matrix 100 wherein aggregates and nanocrystals 101,102, etc. n, n being larger than 102, illustrated by black spots withvariable shape and size, are incorporated. With microscopy measurements,it may be shown that the matrix containing the nanocrystals has mediumrange order between the second and sixth neighboring atoms. Thenanostructure of polymorphous silicon is also notably characterized byinfrared absorption, by microscopy, Raman spectroscopy. It may also becharacterized by a hydrogen exodiffusion spectrum clearly distinct fromthat of amorphous silicon.

For example, differences which may be observed between amorphous siliconand polymorphous silicon are given in FIGS. 4–6.

FIG. 4 illustrates the exodiffusion spectrum for polymorphous silicon.This spectrum is defined by curves illustrating the partial pressure ofhydrogen in millibars, versus the material temperature in ° C. The wayof producing these spectra is well known in the art. Explanations forglobally understanding what is going on here are given hereafter tofacilitate understanding. The partial pressure of hydrogen leaving thematerial is measured versus the annealing temperature. Hydrogen is boundto the material according to different atomic configurations which eachhave a different binding energy. Each binding configuration thereforecorresponds to a hydrogen evolution curve versus temperature, existingas a bell-shaped curve with a peak. The spectrum of amorphous siliconhas the shape illustrated by curve a. It only shows a peak between 500and 600° C., associated with hydrogen being uniformly distributed in theamorphous matrix. Curves b, c, d and e, respectively illustrate hydrogenevolution curves each corresponding to a specific binding configurationof hydrogen. When exodiffusion from polymorphous silicon is measured,curve f is obtained, which corresponds to the resultant of differentbinding configurations of hydrogen which exist in the polymorphousmaterial. The shape of f thereby characterizes incorporation of hydrogenat the surface of aggregates and nanocrystals, and in the matrix havingmedium range order.

Another way of recognizing polymorphous silicon from amorphous siliconwill now be described in conjunction with FIG. 5 which illustrates theinfrared absorption spectrum in the zone for which the wave numberexpressed in cm⁻¹ is between 1,900 and 2,200. Absorption, in anarbitrary unit, is plotted in ordinate and the wave number is plotted inabscissa. Curve d illustrates the experimental result from themeasurement of absorption. Curves a, b, c, respectively illustratecurves obtained by a deconvolution calculation which may be performedsince the different elementary absorption peaks are known. Thisdeconvolution of the experimental spectrum shows the presence of anextra peak p between 2,030 and 2,050 cm⁻¹ for polymorphous silicon. Thispeak corresponds to curve b. The position of the peak depends on theconditions during the making of the polymorphous silicon.

Finally, a diagram of a high resolution image, obtained by transmissionelectron microscopy (HRTEM), of polymorphous material in which crystalswith a diameter of a few nanometers may be distinguished, is illustratedin FIGS. 6A and 6B. These nanocrystals appear on the photograph asregions in which are seen lines parallel to each other. Thesenanocrystals have been illustrated in FIG. 6A by regions inside whichappear dotted lines parallel to each other. A medium range order may beshown by a spatial Fourier transform of the image of the amorphous zone.This transform is schematized in FIG. 6B. Medium range order ismaterialized by the presence of rings surrounding a common point. Foramorphous silicon, two rings and possibly a third very blurred ring maybe seen. For polymorphous silicon, four materialized rings in FIG. 6 bmay be distinguished by the presence of white and black rings. Further,the rings have a greater intensity and a finer width than in the case ofamorphous silicon. On this subject, reference may be made to referencearticle [5] referenced at the end of the description.

A first exemplary embodiment of an assembly of photo detectors accordingto the invention will now be described with reference to FIG. 7. Thisfigure exactly reproduces FIG. 2 from the prior art. In this figure,items with the same reference number as those of FIG. 2 refer to itemshaving the same function, and they may be made in the same way. As forFIG. 7 which reproduces the embodiment of FIG. 2, the only differencebeing that the amorphous material forming the layer i 16 is now replacedwith a layer of polymorphous silicon. Layer n 14 has now been cut outonly allowing electrodes 84 to subsist. Layers i 16 and p 18 arereferenced as 76 and 78, respectively. Contact layer 45 is replaced withcontact layer 85.

A preferred embodiment will now be described in conjunction with FIG. 8.

This embodiment differs from the embodiments of the prior art, inparticular the one described in conjunction with FIG. 2, as in thegeneral case, by the use of a polymorphous material for forming theactive layer on the one hand, but also by the embodiment of theinterconnection layer illustrated at 12′, in FIG. 8, and by the shape ofthe electrodes, two drawings of which are illustrated in FIG. 8 underreference numbers 94 and 64.

In the embodiment described in conjunction with FIGS. 8 or 9, thesubstrate 10 may have any configuration already known from the priorart. This substrate will therefore not be described.

An embodiment of the layer 12′ forming the interconnection brick 12′according to this embodiment of the invention, as well as methods formaking this brick above substrate 10, will now be described.

This layer includes conducting pads 5, 5′ embedded in an insulatedmaterial completely filling side spaces 2, 3, 4 between the pads 5, 5′.

Two embodiments of the interconnection pads are illustrated as 5 and 5′in FIG. 8. These pads are formed above the substrate 10. In the exampleillustrated in FIG. 8, pads 5 and 5′ are cylindrical. The pads may alsobe cubic or conical with their smallest base located on the side of thesubstrate for example, or with bases having different surfaces, forexample hexagonal surfaces. These pads are equal or greater in numberthan the pixels of the imager which one wants to form. On this subject,reference may be made to the reference patent [6] in which an imager isdescribed in conjunction with FIG. 7 of this patent, un whichinterconnection links 56 provide functions other than the functions forconnecting a diode to an addressing circuit. Pads 5, 5′ of theembodiment of the invention described in conjunction with FIG. 8appended to the present application, are formed in aluminium.

Pads 5, 5′ may also be transparent, mono- or multi-layered, with anysizes and geometrical shapes. Their function is to transmit electricalinformation between the substrate 10 and the diode. This notablyrequires proper mechanical adhesion between an interconnection pad 5 andthe lower electrode 64, 94.

Methods for forming the interconnection pad 5 are well known in thefield of microelectronics and optoelectronics. The pads 5 may be madefor example by depositing a continuous conducting layer, by etching cutsaround the pads and by filling the cuts with electrically insulatingmaterial.

Naturally, the layer 12′ may also and preferably be made by firstdepositing the insulating material, then by etching holes at theposition of the pads 5, 5′. The holes are then filled with conductingmaterial(s) forming the pads 5, 5′.

In both production methods which have just been described, the shapeillustrated by layer 12′ in FIG. 8 is obtained, wherein the insulatingmaterial is located in the spaces 2, 3, 4 between the pads 5, 5′. Aplanar surface may then be obtained by optional additionalmechano-chemical polishing.

The shape of the pads 5 results in part from the geometry of the etchingand in part from the etching method.

It is also possible to use lift-off techniques or any other known methodfor obtaining this type of structure.

The materials of pads 5 or 5′ generally are of the metal type likealuminium, copper or tungsten, but it is also possible to use otherconducting materials such as titanium, titanium nitride, a conductingtransparent oxide, a conducting organic material, or any other materialproviding electrical conduction without deteriorating mechanicaladhesion between the pad and the electrode which is located above.Another important characteristic of the pads 5, 5′ is that they do notdeteriorate too much over time by oxidization or electro-migration orunder the effect of temperature. The pad 5 used should notablywithstand, without being deteriorated, the temperature for depositingthe materials forming the detection brick 50, typically a temperature of300° C. for at least 1 hour.

It will be seen later on that depositing the photosensitive materialassumes that the substrate 10 provided with the layer 12′ may besubmitted to this temperature for at least one hour. The diffusionconstant of the material forming a pad 5 should be sufficiently small soas not to compromise insulation of both adjacent pixels upon completionof annealing at 300° C. for one hour. Also, the insulating materialsused in spaces 2, 3, 4 between the pads and in the spaces 7, 8, 9between the electrodes which will be discussed later on, should retaintheir dielectric properties upon completing the same treatment. Thesematerials will preferably be selected in the family of dielectricmaterials used in micro-electronics, for example silicon oxide orsilicon nitride, but not exclusively.

If necessary, it is also possible to use for the material filling theside spaces between the pads and/or between the electrodes, a stack ofdielectric layers in order to produce Bragg mirrors to prevent lightfrom reaching the substrate. Any composite arrangement of material, withany shape or geometry, may be used provided that it meets requirementson electrical insulation, mechanical stability and durability, both overtime and in temperature.

The shape of the electrodes 64, 94 and the method for making them, willnow be described.

It should be understood that in a same imager, the electrodes may allhave the same shape, but not necessarily.

Electrodes 94 and 64 preferably have a shape such that the surface area13 of their zones in contact with a pad 5 is less than the oppositesurface area 13′, which is in contact with the layer 76. The contactsurface area 13 between an electrode 94, 64, and the contact pad 5 maybe less than the surface area of a pad 5 as illustrated for theelectrode 94 in FIG. 8 or on the contrary be equal as illustrated at 64.In the case illustrated at 64, the surface area of the pad 5′ is lessthan the lower surface area 13 of the electrode 64. The surface 13′opposite to the contact surface 13 of the electrode 94 has a shapeproviding a larger electrode surface area in an identical occupiedspace. This means that the upper surface area 13 of the electrode 94 islarger than the surface area of a cross-section 17 of the pixel. Thecross-section of said surface of the pixel is illustrated in dottedlines in FIG. 8. This electrode may be provided with a larger surface bythe concaveness of the electrode 94. Also, in the example illustrated at64, the upper surface 13′ of the electrode has a larger surface areathan the surface occupied by a cross-section of the pixel. Insulatingmaterial completely fills spaces 7, 8, 9 which are in a side positionaround a lower portion of the electrodes.

Current leaks between pads 5, 5′ and a fortiori short circuits may beavoided with the insulating material which fills regions 2, 3, 4 betweenpads 5, 5′ and 7, and 8 and 9 between electrodes 94, 64. Electrodes 94,64 have the purpose of providing electrical contact between thepolymorphous silicon 76 region i and pad 5.

Electrodes 94, 64 are also obtained by standard methods ofmicro-electronics, such as for example by depositing a continuousdielectric layer, etching holes, this etching allowing side spaces 7, 8,and 9 to subsist between the electrodes. Deposition of the conductinglayer(s) forming the material of the electrodes is then performed.

So the cup shape of the electrodes naturally results from the presenceof holes in the etched insulator layer. The electrodes 94, 64 may beseparated from each other by etching the conducting layer or zones 22between electrodes 94, 64.

The electrodes may also and preferably be formed from a stack of layersnotably including, as illustrated in FIGS. 8 or 9, between the layer 76and the electrode 64, 94, a TiN or Ti layer 23 playing the role of adiffusion barrier. Electrode 94 or 64 may also end with a doped siliconlayer or a doped Si_(1−(x+y))Ge_(x)C_(y) (x between 0 and 1; y between 0and 1) alloy surface layer of type n or p, or with a semiconductor layerdoped with ions for example with metal ions, but not exclusively.Electrodes 94 and 64 may also be encapsulated by a layer of organicconductors such as an organic polymer for example.

The materials forming the electrodes 94 and 64 are selected fromconducting materials, provided that they meet the requirements onmechanical, chemical, thermal stability, and stability over time,compatible with the adjacent materials, and the global heat balance ofthe method for making the imager. The materials forming the electrodes64, 94 will notably be selected from aluminium, copper, tungsten,titanium, titanium nitride, a doped semiconductor compatible withneighbouring materials, an organic conductor, a conducting oxide, or anycomposite stack or arrangement of such materials.

Region 76, which will now be described, is the active portion of theelementary photosensitive component; it includes at least a polymorphoussilicon region. Region 76 is an i zone for example if the detectiondiodes are PIN or NIP diodes.

Region 76 preferentially includes an unintentionally doped polymorphouslayer neighbouring a region including silicon and optionally a dopantand optional carbon. Region 76 may contain a gain zone, for example (onthis subject, reference may be made to reference article [7]), dopedzones of different nature and geometry, or any useful arrangement ofmaterial and geometry, provided that it contains at least a regionincluding polymorphous silicon.

In the embodiment illustrated in FIG. 8, a silicon layer 79 is presentat the top of the layer 76 immediately below the layer 24 forming anelectrode. This layer 79 in its upper portion forms the p zone of thediode, by doping silicon with boron. In its lower portion, it mayadvantageously contain carbon. The quality of the interface between thei and p layers is thereby enhanced. This layer 79 may also replace the player 78, appearing above the i layer illustrated in FIG. 7.

Preferably, for a visible light sensor, the sensitive i layer 76 inpolymorphous material will have a thickness between 0.5 and 2 μm.

The region 24 illustrated in FIG. 8 has the purpose, as in the priorart, of forming an electrode for collecting charges, while allowingvisible light to pass through it so that it reaches region 76.Preferably, it will be made in ITO (indium and tin oxide) or based onany conducting transparent oxide or any other transparent and conductingmaterial, including conducting organic polymers.

Surprisingly, it was seen that using polymorphous silicon as a sensitivematerial, led to imagers having lowered image retention in spite of thepresence in the material of a large number of interfaces between thematrix and the nanocrystals and aggregates.

In the embodiment illustrated in FIG. 8, silicon layers 78, 79 arepresent on the top of the layer 76 immediately below a layer 25 formingan electrode which will be discussed later on.

Layer 79 forms the p zone of the diode, by doping silicon with boron.Layer 78 may advantageously contain carbon. Quality of the contact isthereby enhanced.

Preferably, for an ultraviolet imager, the sensitive i layer 76 inpolymorphous material will have a thickness less than 0.4 μm andpreferably around 0.05 μm.

Deposition of the layer 76 in polymorphous silicon will now bedescribed.

Polymorphous silicon is obtained by PECVD (Plasma Enhanced ChemicalVapor Deposition) methods at low temperatures between 100 and 400° C.,from the dissociation of pure silane or mixed with other gases (He, H₂,Ar), under conditions close to forming a powder. Generally, polymorphoussilicon is obtained by adjusting technological parameters of the plasmafrom which deposition is made: pressure, dilution, and radiofrequencypower, which result in the formation of aggregates and nanocrystals inthe plasma. On this subject, reference may be made to reference article[5]. The polymorphous material is therefore formed from theincorporation of aggregates and nanocrystals which impart topolymorphous silicon its specific properties, particularly adapted todetection. Indeed, in spite of the microstructure, the presence ofaggregates and nanocrystals, induces a low density of defect states anda large product mu tau (carrier mobility times carrier lifetime)typically 100 times larger than that for amorphous silicon.

FIG. 10 illustrates the mu tau product obtained on metal-(pm-Si:H)-metalstructures versus wavelength,. Curve 1 is the response of a thick sampleof polymorphous material (of the order of 1.5 μm) which has lowselectivity (about 10). The thickness of the polymorphous layer wasreduced in order to increase the ratio of UV to visible lightselectivity. Curve 2 shows the response of a polymorphous layer with athickness of the order of 200 Å. This curve shows a substantiallyconstant mu tau product between wavelengths of 250 and 500 nanometers.Wavelength selectivity for this layer will therefore be low. Curve 3 isthe response of a thin layer of polymorphous silicon with a thicknessidentical with that of curve 2 but which differs from the latter by thedeposition conditions. This curve shows a strong decrease of the mu tauproduct when passing from a wavelength of 200 nanometers to a wavelengthof 400 nanometers, so it is possible to obtain selectivity among thedifferent wavelengths favouring ultraviolet wavelengths. A selectivityof the order of 38,000 is obtained.

The upper electrode 25 illustrated in FIG. 9 for the ultraviolet imagermay, as in reference document [2], have the shape of a metal grid.

It may also as illustrated in FIG. 11, exist as two combs 27, 28including interdigitated teeth 27′, 28′ respectively. Both combs areisolated from each other so that they may be set at differentpotentials. Finally, electrode 25 may also exist as a thin silver layer.Reference article [6] and notably its FIG. 3 show different spectralresponses depending on the thickness of the silver layer forming anupper electrode. A wavelength peak around 320 nanometers may be obtainedwith the silver layer, the bandwidth from this peak being reduced whenthe thickness of the silver layer varies between 10 nanometers and 130nanometers for example.

It should be specified that in the embodiment of the detection brick 50which has just been described, one PIN diode is formed per pixel. One ofthe contacts of the PIN diode is formed by the upper surface of pad 5,5′. Electrodes 64, 94 may be made in an n doped semiconductor, forexample in amorphous hydrogenated carbonaceous silicon a-SiC:H, dopedwith arsenic. The intrinsic layer is formed with hydrogenated siliconpm-Si:H. The p layer 79 is again hydrogenated amorphous silicon a-Si:H,doped with boron and optionally with carbon.

Alternative embodiments of the detection brick 50 will now be described.These alternatives independently from each other, relate to variationson the layer 76 and on the electrode 25.

Instead of being in the n-i-p order as described above, the stacking oflayers forming each of the diodes above pads 5, 5′ may also be p-i-n.

It is not mandatory to have electrodes 64, 94 above each pad 5, 5′. Abarrier may be obtained by directly depositing an intrinsic layer i,unintentionally doped, directly above the interconnection brick 12′. Inthis case, at least the upper layer of the pads 5, 5′ will be a metal,for example platinum, tungsten or palladium. An n layer or a p layer maybe located above layer i.

The stacking of layers above pads 5, 5′ may also comprise a p layer oran n layer, then layer i, this i layer being directly in contact withthe electrode 25, which, in this case, is metal, for example platinum,tungsten or palladium.

In all the cases which have just been described above, layer i and atleast a sublayer of this i layer, are in polymorphous silicon.

Surprisingly, it was seen that using polymorphous silicon as sensitivematerial led to imagers having reduced image retention and very highsensitivity.

FIG. 12 shows the gain in performance obtained on a device according tothe invention. This figure illustrates two curves a and b. Each of thecurves illustrates the dark current remaining for 0.2 seconds afterinterruption of incident light, with the optical power plotted inabscissa. The curve a illustrates this dark current for hydrogenatedamorphous silicon, and curve b shows this same current for a layer usingpolymorphous silicon according to the invention. It is seen that thedark current fraction after light extinction is substantially lower withpolymorphous silicon than with amorphous silicon. It follows that theimage retention effect obtained with an imager according to theinvention is substantially reduced by using a pixel based on apolymorphous material.

It was further seen that with polymorphous material time stability ofimagers may notably be increased, especially under illumination. Withpolymorphous material, dark leakage currents of the order of 10⁻¹¹ A/cm²may be attained with excellent spectral response. The measured externalquantum efficiencies attain more than 30% in the vicinity of theincident wavelength of 300 nm, and the width of the forbidden gap forpolymorphous silicon is greater than that for amorphous silicon, so theobtained photocurrent in the vicinity of 700 nm, i.e., in red light andnear infrared light, is lower, which has the advantage of no longerrequiring coloured filters for eliminating infrared parasitic radiation,as compared with crystalline silicon CMOS technologies.

The advantage of the low dark leakage current is maintained regardlessof the size of the pixel. Measurements made by the applicant andillustrated on the graph of FIG. 13, show that with polymorphoussilicon, the points representing the dark leakage current versus thesize of the pixel have substantially the same ordinate: reducing thesize of the pixel has little influence on its dark leakage current.

LIST OF QUOTED DOCUMENTS

[1] U.S. Pat. No. 6,114,739

[2] Article of M. Razeghi and A. Rogalski entitled “Semiconductorultraviolet detectors”; Applied Physics Reviews; May 15, 1996; pages7433–7473.

[3] U.S. Pat. No. 5,682,037

[4] Article from the Journal of Applied Physics, volume 86, No. 2, of R.Meaudre et al., pages 946–950, entitled “Midgap density of states inhydrogenated polymorphous silicon”

[5] Article from the Journal of Non-crystalline Solids 299-302 (2002)pages 284–289 of A. Fontcuberta et al.

[6] U.S. Pat. No. 6,018,187

[7] Article from the Journal of Applied Physics, volume 87, No. 4 of R.Vanderhaghen et al., pages 1874–1881, entitled “The origin of currentgain under illumination in amorphous silicon n-i-p-i-n structures”.

[8] Article of Marko Topic et al.—Applied Physics Letters—volume 78—No.16, p. 2387–2389.

1. An assembly of sensors formed as an imager each sensor delivering asignal corresponding to one pixel of the image, and having, a detectionbrick with a detection zone including a photosensitive material, a brickfor addressing and optionally processing signals from the sensors, thisbrick notably bearing an addressing circuit and, an interconnectionbrick located between the detection brick and the addressing brick, eachbrick bearing connection pads connecting the sensors of the imager tothe addressing circuit, so that the signals from the sensors areindividualized, characterized in that the photosensitive material of thedetection brick contains at least one polymorphous silicon layer.
 2. Theassembly of the sensors forming an imager, according to claim 1,characterized in that the polymorphous silicon layer has a thicknessless than 4,000 angstroms.
 3. The assembly of the sensors forming animager, according to claim 1, characterized in that the interconnectionbrick is formed by pads (5, 5′) embedded in an insulator (1, 2, 3). 4.The assembly of the sensors forming an imager, according to claim 1,characterized in that the interconnection pads (5, 5′) are either inaluminium or copper or tungsten or chromium.
 5. The assembly of thesensors forming an imager, according to claim 3, characterized in thatthe insulating material embedding the pads, is formed by a stack ofdielectric layers forming Bragg mirrors.
 6. The assembly of the sensorsforming an imager, according to claim 3, characterized in thatelectrodes (64, 94) are formed above the pads (5, 5′), these electrodeshaving a lower surface electrically coupled with a pad (5) and an uppersurface, the upper surface of the electrode having a larger surfacedimension than the lower surface in contact with the pad.
 7. Theassembly of the sensors forming an imager, according to claim 6,characterized in that the upper surface of the electrodes is cup-shaped.8. The assembly of the sensors forming an imager, according to claim 6,characterized in that a lower portion of each electrode is embedded inan insulator layer, an upper portion of this electrode being just abovesaid insulator layer.
 9. The assembly of the sensors forming an imager,according to claim 8, characterized in that the insulator layersurrounding a lower portion of the electrodes consists of a stack oflayers forming Bragg mirrors.
 10. The assembly of the sensors forming animager, according to claim 6, characterized in that the electrodes (64,94) are either in aluminium or copper or tungsten or titanium orchromium or titanium nitride or a doped semiconductor or an organicconductor or even finally a composite stack of the aforementionedmaterials.
 11. The assembly of the sensors forming an imager, accordingto claim 6, characterized in that the polymorphous silicon layer (76) isplaced above the layer including the insulator and the electrodes. 12.The assembly of the sensors forming an imager, according to claim 11,characterized in that at least one upper silicon layer (79) has a lowerportion containing carbon and an upper portion containing boron.
 13. Theassembly of the sensors forming an imager, according to claim 1,characterized in that the thickness of the polymorphous silicon layer(46) is between 0.5 and 2 μm.
 14. The assembly of the sensors forming animager, according to claim 1, characterized in that the detection zoneincluding the polymorphous silicon is an intrinsic zone of a PIN or NIPdiode.
 15. The assembly of the sensors forming an imager, according toclaim 12, characterized in that electrodes 64, 94, are formed above pads5, 5′, and these electrodes being etched in an n or p material layer.16. The assembly of the sensors forming an imager, according to claim10, characterized in that at least one of the electrodes contains an ndoped material.
 17. The assembly of the sensors forming an imager,according to claim 10, characterized in that at least one of theelectrodes contains a p doped material.
 18. The assembly of the sensorsforming an imager, according to claim 9, characterized in that at leastone of the electrodes contains an n doped material.
 19. The assembly ofthe sensors forming an imager, according to claim 1, characterized inthat the polymorphous material layer is an intrinsic layer placed abovethe electrodes.
 20. The assembly of the sensors forming an imager,according to claim 2, characterized in that the polymorphous materiallayer is an intrinsic layer placed above the electrodes.
 21. Theassembly of the sensors forming an imager, according to claim 12,characterized in that the polymorphous material layer is an intrinsiclayer placed above the electrodes.
 22. The assembly of the sensorsforming an imager, according to claim 19, characterized in that a pdoped layer is placed above the amorphous silicon layer, therebyproducing a NIP diode.
 23. The assembly of the sensors forming animager, according to claim 20, characterized in that a p doped layer isplaced above the amorphous silicon layer, thereby producing a NIP diode.24. The assembly of the sensors forming an imager, according to claim21, characterized in that a p doped layer is placed above the amorphoussilicon layer, thereby producing a NIP diode.
 25. The assembly of thesensors forming an imager, according to claim 19, characterized in thatan n doped layer is placed above the intrinsic amorphous silicon layerthereby producing a PIN diode.
 26. The assembly of the sensors formingan imager, according to claim 20, characterized in that an n doped layeris placed above the intrinsic amorphous silicon layer thereby producinga PIN diode.
 27. The assembly of the sensors forming an imager,according to claim 21, characterized in that an n doped layer is placedabove the intrinsic amorphous silicon layer thereby producing a PINdiode.
 28. The assembly of the sensors forming an imager, according toclaim 6, characterized in that the pads include an upper metal surface,and in that the polymorphous material layer is directly placed incontact with the pads.
 29. The assembly of the sensors forming animager, according to claim 28, characterized in that an n doped layer isplaced above the polymorphous silicon layer.
 30. The assembly of thesensors forming an imager, according to claim 28, characterized in thata p doped layer is placed above the polymorphous silicon layer.
 31. Theassembly of the sensors forming an imager, according to claim 30,characterized in that the electrode is in conducting transparent oxide.32. The assembly of the sensors forming an imager, according to claim30, characterized in that the electrode is produced in a layer of ametal partly transparent to ultraviolet radiation.
 33. The assembly ofthe sensors forming an imager, according to claim 30, characterized inthat the upper electrode is a metal grid.
 34. The assembly of thesensors forming an imager, according to claim 30, characterized in thatthe electrode is formed by two combs each having teeth, the teeth beinginterdigitated.
 35. A method for producing an assembly ofphoto-detectors according to claim 6, characterized in that: afterproducing a substrate notably including an addressing circuit andoptionally signal processing circuits, one or several layers ofinsulating materials are deposited, said layer is etched so as to formholes in this layer, some holes are filled with a conducting materialthereby forming interconnection pads (5, 5′), mechano-chemical polishingis carried out optionally, one or more insulating material sublayers aredeposited, said insulating material sublayer is etched above pads (5,5′), a layer is deposited above the etched insulator layer, therebyforming a non-planar conducting material layer, cups appearing above thepads, the conducting material layer which has just been deposited, isetched, in order to form electrodes separated from each other, anunintentionally doped polymorphous material layer is deposited, a dopedlayer is deposited, a conducting material (24) layer forming an upperelectrode is deposited finally.
 36. The method according to claim 35,characterized in that the temperature for depositing the polymorphousmaterial is between 175° C. and 250° C.
 37. The method according toclaim 35, characterized in that the polymorphous material deposit isfollowed by a deposit of a layer containing carbon at least in its lowerportion.
 38. The method for producing an assembly of sensors accordingto claim 3, characterized in that: the method comprises a step fordepositing a polymorphous silicon layer, this layer coming into contactwith either a metal upper portion of conducting pads, or n doped or pdoped electrodes, themselves in contact with a conducting pad, this stepfor depositing the polymorphous silicon layer being produced by a PECVD(Placement Enhanced Chemical Vapor Deposition) method, and at atemperature between 150 and 250° C.